1. Field of the Invention
The present relates to a wireless terminal device, and particularly a wireless terminal device which is provided with a direct conversion receiving circuit using an even harmonic mixer.
2. Description of the Background Art
At present, wireless terminal devices such as a cellular telephone primarily employ receiving circuits of a heterodyne type. Although the heterodyne type requires an intermediate frequency circuit, a local oscillation signal does not leak toward an antenna, and therefore, deterioration in sensitivity due to the leakage does not occur because a local oscillation frequency is different from a receiving frequency.
In contrast to the above, the direct conversion (homodyne) type does not require an intermediate frequency circuit, but uses the local oscillation frequency equal to the receiving frequency. Therefore, the local oscillation signal may leak toward the antenna, and thereby the sensitivity may be deteriorated.
According to the heterodyne type, the intermediate frequency signal in the intermediate frequency circuit is converted to a base band signal of a low frequency, and is applied to a base band circuit. The base band signal applied to the base band circuit is kept at a constant level by a variable gain amplifier in the intermediate frequency circuit. Therefore, the level of the base band signal hardly varies in the base band circuit.
In contrast to this, the direct conversion type does not use the intermediate frequency circuit, and the received radio-frequency signal is directly converted to the base band signal of a low frequency. Therefore, the levels of the base band signal applied to the base band circuit varies when the level of the received radio-frequency signal varies.
In general, when an input signal is applied to a circuit element, an output signal contains an unnecessary DC offset component in addition to a desired signal component. According to the heterodyne type described above, since the base band signal applied to the base band circuit is at a constant level, the DC offset component contained in the output signal is constant. Therefore, the base band circuit can be designed in view of such a constant DC offset component.
According to the direct conversion type, however, the level of the base band signal applied to the base band circuit is variable. Therefore, the DC offset component contained in the output signal of the circuit element in the base band circuit is not constant, and varies to a relatively large extent. It is very difficult to design the base band circuit in view of this variation. Further, the operation of the base band circuit may be saturated due to the fact that the base band signal applied to the base band circuit contains the DC offset component.
Various manners and systems have been disclosed for preventing an influence of the DC offset component in the DC conversion system (Japanese Patent Laying-Open Nos. 10-233711, 10-13482, 9-83595 and 10-56394). However, these manners are different in principle from a manner or method according to the invention, which will be described later.
The base band signal has a low frequency, and is similar to a DC (which can be considered as an AC having a frequency of 0 Hz). Therefore, if such a manner is employed that merely removes the DC offset component, this also removes the signal component of the base band signal. If the cut-off frequency is lowered toward 0 Hz as far as possible, the signal component is hardly removed, but a transition response becomes slow.
This results in deterioration of operation characteristics of the base band circuit.
An object of the invention is to provide a wireless terminal device, in which only a DC offset component can be removed without slowing a transition response.
According to the invention, a wireless terminal device includes an antenna, and a receiving circuit for receiving a received radio-frequency signal from the antenna. The receiving circuit includes a local oscillator, an even harmonic mixer, a first high-pass filter, a variable gain amplifier and a second high-pass filter. The even harmonic mixer mixes the received radio-frequency signal with a local oscillation signal supplied from the local oscillator to produce a base band signal. The first high-pass filter receives the base band signal from the even harmonic mixer, and has a first cut-off frequency. The variable gain amplifier amplifies the first base band signal passed through the first high-pass filter to obtain a constant output level. The second high-pass filter receives the base band signal from the variable gain amplifier, and has a second cut-off frequency higher than the first cut-off frequency.
In the wireless terminal device described above, the first high-pass filter removes the DC offset component occurred in the even harmonic mixer. Since the first high-pass filter has a lower cut-off frequency than the second high-pass filter, only the DC offset component can be reliably removed without substantially removing the signal component. Since the first high-pass filter has a low cut-off frequency, its transition response is slow, but the circuit operation speed does not decrease because the DC offset component occurred in the even harmonic mixer is constant even when the level of the received radio-frequency signal varies. Meanwhile, the DC offset component occurred in the variable gain amplifier is removed by the second high-pass filter. Although the level of the DC offset component occurred in the variable gain amplifier varies, this DC offset component can be reliably removed with a high transition response because the second high-pass filter has a higher frequency than the first high-pass filter.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.